1. Field of the Invention
The present invention relates to a novel blister resistant pressure sensitive adhesive sheet. More particularly, the present invention relates to a novel blister resistant pressure sensitive adhesive sheet which has excellent resistance to formation of blisters and excellent removability on heating and after heating and is suitably applied to molded plastic products, preferably to polycarbonate plates.
2. Description of the Prior Art
As is well known, pressure sensitive adhesive sheets having a coating layer of a pressure sensitive adhesive are widely used and now indispensable in a variety of industrial fields, for example, as a printed label, seal on wrapped packages and the like. This is because the pressure sensitive adhesive sheets can be readily bonded to the surface of various substrates conveniently by merely putting on and lightly pressing to the substrate surface and, different from adhesives in general, the pressure sensitive adhesive forming the coating layer of the sheet contains no organic solvent so that no problem of environmental pollution are caused thereby.
Pressure sensitive adhesive sheets can be applied and bonded to the surface of a substrate article of any materials almost without limitation including paper, wood, metals, ceramics, plastics and the like. As a consequence of the trend in recent years that light weight articles are preferred, the quantity of plastic products is rapidly increasing year by year. Thus, the occasion in which pressure sensitive adhesive sheets are attached to plastic goods or plastic packages of goods as a label is also increasing rapidly. When a transparent pressure sensitive adhesive sheet used in such an application has a printed pattern as desired, the plastic article to which such sheet is attached on the surface may have an outer appearance as if it were printed directly thereon.
Apart from the application as a label or seal of packages, pressure sensitive adhesive sheets are sometimes attached to the surface of a board of polycarbonate resins used as a window pane by virtue of the high transparency with an object to protect the surface from scratches or to adjust transmission of sun light therethrough.
Pressure sensitive adhesive sheets in general have a serious problem that, when a pressure sensitive adhesive sheet is applied and bonded to the surface of an article made of polystyrene resin, polycarbonate resin and the like, so-called blisters are not rarely formed due to incomplete squeeze out of the gas between the substrate surface and the pressure sensitive adhesive sheet attached as a label or a protecting sheet. This problem has been unavoidable when the face stock film of the pressure sensitive adhesive sheet is made of a resin having gas barrier property such as conventional polyester resins.
When blisters are caused in the presence sensitive adhesive sheet attached to the surface of an article as a label, not only the article has a very unsightly appearance to decrease the commercial value but also the label and the like attached to the surface are liable to eventual falling. In a printed label made of a see-through pressure sensitive adhesive sheet, in particular, it is unavoidable that the appearance is extremely poor because the layer of the adhesive coating exhibits a pockmarked or white and cloudy appearance through the face-stock film.
The mechanism of this undesirable phenomenon is that the plastic film as the face stock film of the pressure sensitive adhesive sheet acts as a barrier against escape of the gas in a small volume occurring between the face stock film and adhesive coating layer so that the gas retained there forms a number of gas-filled blisters.
One of the possible means to solve this problem by dissipating the gas in the blisters is to provide a gas permeable layer made of paper, porous polyurethane and the like between the face stock film and the adhesive coating layer of the pressure sensitive adhesive sheet. This method, however, has no general applicability because the overall thickness of the pressure sensitive adhesive sheet is necessarily increased so much and the pressure sensitive adhesive sheet can no longer be used as a see-through label due to the loss of transparency by the interposed gas permeable layer.
It is also known that the problem due to the formation of blisters in a pressure sensitive adhesive sheet can be considerably decreased when an adhesive of a certain type is used for the adhesive coating layer. Examples of such an adhesive hitherto proposed include those rubber-based ones comprising a block copolymer of styrene-isoprene-styrene type compounded in a specific formulation. Pressure sensitive adhesive sheets having an adhesive coating layer of such a rubber-based adhesive are, however, not quite satisfactory because of the poor weatherability of the adhesive layer inherent in rubber-based pressure sensitive adhesives.
It has been known for a long time that novel functions can be exhibited by a polymer by grafting a different kind of polymer to the main chain of the polymer ("Polymer Alloys", edited by the Society of Polymer Science, Japan and published by Tokyo Kagaku Dojin in 1981; and Nippon Gomu Kyokaishi, Volume 59, Page 331, 1986).
In the field of adhesive materials, adhesives having well balanced adhesive properties, such as tackiness, adhesive strength and holding power, can be prepared by using as the main component thereof a copolymer having a main chain of acrylic units and side chains of a high glass transition temperature (Tg) grafted to the main chain. One of the methods of grafting the side chains is reaction of functional groups attached to the main chain with a polymer which is of different kind from the main chain and has reactive terminal functional groups with the functional groups of the main chain (Laid-open Japanese Patent Publication Showa 56-59882). Another of the methods of grafting the side chain is a method of using macromonomers (Laid-open Japanese Patent Publication Showa 59-75975; U.S. Pat. No. 4,551,388; and Adhesive Age, Volume 28, No. 9, Page 17, (1985)).
Macromonomers having a Tg of below 0.degree. C. can be utilized in the method of using macromonomers (Laid-open Japanese Patent Publication Showa 62-288676; Laid-open Japanese Patent Publication Showa 63-196676; Laid-open Japanese Patent Publication Heisei 1-146977; and Laid-open Japanese Patent Publication Heisei 1-203412) as well as the macromonomer having a Tg of above room temperature which is described above.
The present inventors previously studied extensively to develop pressure sensitive adhesive sheets having excellent adhesive strength and excellent resistance against formation of blisters simultaneously and it was discovered that a pressure sensitive adhesive sheet comprising a coating layer which comprises as the main component thereof a copolymer having a specific molecular weight and a specific Tg and comprising monomer units of acrylic ester, monomer units of polar acrylic compound and macromonomer units having polymeric chain and terminal polymerizable functional groups in a specific ratio is suitable for the object.
In practical applications of pressure sensitive adhesive sheets, it is also very important that, when necessary, the pressure sensitive adhesive sheets are easily removed from the surface of the substrate to which they are applied without any remaining adhesive material on the substrate. Heating is one of the practical methods to facilitate the removal of pressure sensitive adhesive sheets and it is important that pressure sensitive adhesive sheets have excellent removability on heating and after heating.
The pressure sensitive adhesive sheet thus discovered previously by the present inventors does not always have satisfactory removability on heating and after heating even though it has excellent adhesive strength and excellent resistance against formation of blisters simultaneously.